Dual-Mounted Setup Idea

[Shibby]

Having been unable to sell my old Atik 428ex, I've been casually thinking about the possibility of using it in a dual imaging rig.

I may or may not pursue this, but thought I'd ask the more knowledgeable folk here for their thoughts and to sanity check this (possibly foolish) idea.

I currently have a 190MN mounted on a Mach1GTO with a 50mm finder/guider. I image with a 460ex. My idea is to side-by-side this with a smaller scope that has roughly half the focal length of the main scope, then use this to capture the RGB for my targets at the same time as luminance / narrowband with the 190MN. The pixel size of the 460ex and 428ex is the same. Something like a Sharpstar 76EDPH might fit the bill, although it's FL is 418mm. The smaller sensor shouldn't give an FOV issue given the shorter focal length.

I understand that the resolution would be halved, but figure this is okay for RGB. I also understand that the smaller aperture means longer exposure times than if I binned the RGB with the 190MN, but at least I would be doing it in parallel with luminance. Does this make sense - would it be worthwhile?

I've read that flexure is a real problem with guiding on a dual setup. Given that the guider is mounted on the larger scope, would I be okay if I manage to keep the smaller scope as lightweight as possible? Is side-by-side the only way to go? Will the extra weight on my mount cause guiding issues despite how it's mounted?

If the above just doesn't work, perhaps I'd be better off going for widefield images with much smaller optics. However, it's a shame the sensor on the 428ex will limit just how wide that field would be.

[SamAndrew]

I'd think you can go for a piggy back setup with a scope as small as the 76; people piggy back larger guide scopes.

You've got the added complication of running 2 auto focusers and 2 filter wheels (lots of usb ports needed, or troublesome hubs). You need to think about timing exposures, dithers, and auto focus runs. Depends which program you use; there is dual camera support in APT, but SGP still lacking afaik.

If you have any data from a small refractor you can try combining it to see how it turns out, or ask for some raw data from someone else. My personal experience trying to add Quattro 8S luminance to RGB from my star71 didn't turn out well, the field curvature wasn't the same so the alignment wasn't great, resulting in stars that were different colurs on each side. I'm sure it can be done, but it's just an extra headache to sort out and I gave up pretty quickly!

A lot of extra headaches that just suck the fun out of it! I've gone down the Rasa route to aquire more data that way.

[Shibby]

Thanks very much for your comments @SamAndrew Hmm, you might well be right in terms of the risk to "fun"... I really like how, at the moment, I can be up and running in 10 minutes and everything works nicely.

As for combining data, did you try Registar when you were doing it? I know it can account for distortion etc so should solve that problem.

Anyway, perhaps I should think about another use for my 428ex or try again to shift it.

[tomato]

You might be ok piggy backing the smaller scope on the MN 190 but I would go with the side by side option if it is doable, and keep the guidescope on the MN190. The sharpstar/428 has a significantly larger FOV so you wouldn’t need an adjustable saddle if you are going to work with the MN190/460 FOV.

If you are keeping the MN190 just for Lum and Ha, you could just screw or fit the filters into an adapter, that’s what I do rather than run two filter wheels. I therefore capture 3x more Lum than the RGB channels, if I dither I run two instances of NINA on the same machine and use the synchronised PHD option. This waits for both subs to finish before performing the dither. 
I have combined RGB from a OSC CMOS with L from a CCD with reasonable success, but I managed to get the FOVs and imaging scale reasonably similar. APP combines the data, no problem.

I can’t guarantee success with your proposal, but I reckon it would be fun to try, although I guess it depends on how much tinkering you like to do. 

[SamAndrew]

On 20/02/2022 at 20:00, Shibby said:

As for combining data, did you try Registar when you were doing it? I know it can account for distortion etc so should solve that problem.

I don't have a copy of Registar unfortunately, I used pixinsight; I might have another go, I remember I had to manually match some of the stars in PI as the scales were so different.

[ollypenrice]

13 hours ago, SamAndrew said:

I don't have a copy of Registar unfortunately, I used pixinsight; I might have another go, I remember I had to manually match some of the stars in PI as the scales were so different.

I wouldn't attempt any mixing of twin-scope data without Registar. It's fairly expensive, it's a one trick pony, but it's a darned good trick! I've done a lot of dual rig imaging with twin Tak FSQ and twin TEC140 and there's a lot to be said for it. However, the finer your pixel scale is, the harder it is to avoid trailing from flexure on one scope. This would probably be less of an issue with a small refractor and you could get away with a less costly alignment device. I have always used the Cassady T-Gad which is no longer in production and was brutally expensive even second hand. FLO have possible candidates.  Unless one side has a much larger FOV than the other, you'll need some kind of alignment device.

Interesting reading tomato's solution to the dither problem. My solution was not to dither and rely on the different noise profiles of the two sides. With the low noise of modern cameras I don't know how import dither is, in any case. I don't miss it. 

At best, when both rigs are the same, your maximum benefit from a dual rig is, by definition, one F stop. Something like the RASA blows that out of the water. However, I've used two dual rigs and one RASA and the twin Tak dual rig was easier, though a lot slower, than the RASA. The twin TEC at high res has always been difficult.

Finally, we have to be careful in comparing small-scope RGB data with large-scope binned RGB even when the pixel scale is comparable. The signal, however, will be far weaker in the unbinned small-scope data. I suspect that adding at least some RGB from the main scope would probably be necessary.  I can't be sure because it's always been a principle on our dual rigs to use the same scopes and comparable cameras on both sides.

Olly

[Shibby]

Thanks all. Its given me plenty of food for thought!
 

On 20/02/2022 at 21:00, tomato said:

I have combined RGB from a OSC CMOS with L from a CCD with reasonable success

What setup do you have @tomato? Are you using a side-by-side rig currently?

4 hours ago, ollypenrice said:

With the low noise of modern cameras I don't know how import dither is, in any case. I don't miss it. 

I also don't dither yet, despite it being highly recommended by many. Perhaps my guiding isn't accurate enough to make it a necessity? (Although I do typically have an RMS ~0.5"). I often combine data from multiple sessions so I suppose that makes a difference.
 

4 hours ago, ollypenrice said:

The signal, however, will be far weaker in the unbinned small-scope data

Indeed - the smaller aperture is going to mean I wouldn't get away with shorter exposures for RGB as I do now. The overall benefits seem to be diminishing the more I explore the idea.

If my spare camera was a large-sensor OSC I might instead consider using it to capture some "context" images to show alongside the close-ups. Or just to have the option of wide/narrow field on the night.

[tomato]

11 hours ago, Shibby said:

Thanks all. Its given me plenty of food for thought!
 

What setup do you have @tomato? Are you using a side-by-side rig currently?

I have a dual side by side Esprit 150 rig, which can run with a variety of cameras. 
They are mounted on a Mesu 200 with an ADM adjustable saddle on the RH scope to facilitate sensor alignment. The bracing plate across the top of the tube clamps is essential to minimise differential flexture.

[Tomatobro]

I went the for the larger field smaller optics route using two Samyang 135 F2 lenses. The cameras are Atik Horizon 2 OSC and ZWO ASI1600mm with filters

Getting the field of view to match was the only real issue as differential flex turned out not to be a problem.  Weight wise it was about the same as my MN190

I just spotted that the toothed belt on the focus stepper motor is not fitted on the ZWO scope

Edited February 26, 2022 by Tomatobro
update

[gorann]

18 hours ago, tomato said:

I have a dual side by side Esprit 150 rig, which can run with a variety of cameras. 
They are mounted on a Mesu 200 with an ADM adjustable saddle on the RH scope to facilitate sensor alignment. The bracing plate across the top of the tube clamps is essential to minimise differential flexture.

Looks great  Steve! That is the setup I will build this summer once Olly's old Mesu gets updated and delivered here. But what are those odd looking things at the ends of the scopes, attached with aluminium foil. They are not round and red😉

Edited February 26, 2022 by gorann

[tomato]

11 hours ago, gorann said:

Looks great  Steve! That is the setup I will build this summer once Olly's old Mesu gets updated and delivered here. But what are those odd looking things at the ends of the scopes, attached with aluminium foil. They are not round and red😉

Thanks Gorann. Hiding under the foil are two ASI 178s, retro fitted with Peltier coolers by @Tomatobro. These are my sensor of choice for small galaxy hunting, I have had some success with this setup but still trying to optimise SNR vs resolution. I’m currently trying 3x3 binning…

I look forward to your Esprit rig, I can send you details of the top bracing plate which has been quite successful at controlling the differential flexture problem associated with such a heavy set up, but if you have your own ideas I’ll follow them with great interest.

[gorann]

2 hours ago, tomato said:

Thanks Gorann. Hiding under the foil are two ASI 178s, retro fitted with Peltier coolers by @Tomatobro. These are my sensor of choice for small galaxy hunting, I have had some success with this setup but still trying to optimise SNR vs resolution. I’m currently trying 3x3 binning…

I look forward to your Esprit rig, I can send you details of the top bracing plate which has been quite successful at controlling the differential flexture problem associated with such a heavy set up, but if you have your own ideas I’ll follow them with great interest.

Actually I have no clear indication of flexing with the RASAs, possibly because they are rather short and because the FL in only 400 mm. A dual Esprit 150 rig is clearly more of a challenge and a bracing plate is probably a must. I will probably ask for your advice when I start building it in the summer.